The instant invention is directed to a process of alumina manufacture via the Bayer process. The Bayer process is the almost universally used process for the manufacture of alumina. In its broadest aspects, this method is carried out almost exclusively in aqueous solution, and is achieved by reaction of bauxite and a strong base such as caustic soda or lime in steam heated autoclaves whereby the alumina is transformed into a soluble aluminate form. In this step, a considerable amount of insoluble impurities results or is released from the bauxite, which recrement must be separated from the desired alumina constituent. These residues commonly known as red muds include iron oxides, sodium aluminosilicate, titanium oxide and other materials. Generally these muds appear as very fine particles which are difficult to separate out. Yet the red muds which usually constitute about 10 to 50% by weight of the ore must be rapidly and cleanly separated from the solubilized alumina liquor in order to make this particular step economically efficient. If the rate of separation is too slow, output is materially diminished and overall process efficiency impaired. Likewise, if the separation is not clean, the resultant alumina in the form of aluminate is somewhat crude and undesirable for a number of end-uses. The insoluble impurities present in the alumina as carry-through from the manufacturing process tend to add extraneous non-active matter into the specific media, such as water, being treated with aluminate for a variety of purposes. For example, low grade sodium aluminate containing relatively large amounts of mud impurities when used to treat water results in a situation of increased tendency to form slime masses as the direct result of the insoluble impurities present, which masses tend to foul feeding equipment. Also, if the crude sodium aluminate contains substantial amounts of impurities as an admixture, solution problems are quite difficult to overcome if the aluminate is fed in the form of a solid.
One method of overcoming the above problems, and materially speeding up separation of red muds from alumina as well as effecting a cleaner separation of the constituents is disclosed in U.S. Pat. No. 3,390,959 wherein it is disclosed that the use of homo- or copolymers of acrylic acid and acrylates which contain not more than 20% of other ethylenically unsaturated polymerizable polar monomers as red mud flocculants improves the overall efficiency of the Bayer process.
The '959 patent teaches the use of copolymers of acrylic acid or acrylates with ethylenically unsaturated monomers as red mud flocculants, it likewise discloses, however, that when the ethylenically unsaturated monomers constitute more than 5 molar percent the separation rate dramatically drops and at more than 20 molar percent no significant separation is obtained. Although this teaching is accurate for the initial or "head" stage of the recovery circuit, it has surprisingly been found not to apply to the results obtained at the latter, more dilute stages of the caustic recovery circuit.